This invention relates generally to imaging systems and more particularly to systems and methods for improving a resolution of an image.
A computed tomography (CT) imaging system typically includes an x-ray source that projects a fan-shaped x-ray beam through a patient to an array of radiation detectors. The beam is collimated to lie within an xy plane, generally referred to as an “imaging plane”. Intensity of radiation from the beam received at the detector array is dependent upon attenuation of the beam by the patient. Attenuation measurements from a plurality of detector cells of the detector array are acquired separately to produce a transmission profile.
The x-ray source and the detector array are rotated within a gantry and around the patient to be imaged so that a projection angle at which the beam intersects the patient constantly changes. A group of x-ray attenuation measurements, which is analog projection data, from the detector array at one gantry angle or one projection angle is referred to as a “view”. A “scan” of the patient includes a set of views made at varying projection angles, during one revolution of the x-ray source and detector array.
To reduce a total scan time used to acquire multiple slices, a helical scan may be performed. Helical scan techniques allow for large volumes to be scanned at a quicker rate using a single photon source. To perform the helical scan, a table on which the patient rests, is moved along a z-axis about which the gantry rotates while analog projection data for a prescribed number of slices is acquired. The helical scan generates a single helix. The helix mapped out by the beam yields analog projection data from which images in each prescribed slice may be reconstructed. In addition to reducing scan time, the helical scan provides other advantages such as better use of injected contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images. An example of the helical scan includes a multi-slice helical scan. In the multi-slice helical scan, the detector array extends along the z-axis. Typically, in the multi-slice helical scan, the detector array contains multiple rows, with each row corresponding to a different position along the z-axis, and a different measured slice. In an axial scan, analog projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the patient. For discrete slices, iterative reconstruction of a full field of view may be performed to increase image quality.
For continuous scans, a scan pattern in which a position of the patient along the z-axis varies linearly with a rotation of the gantry is produced. During data acquisition, the continuous scan pattern is subject to quantization, and a discrete set of projection views is generated for a limited number of positions of the x-ray source around the patient. Conventional direct image reconstruction techniques, such as two-dimensional or three-dimensional filtered back-projection, reconstruct image voxels from projection data by interpolating elements in the projection data to accumulate contributions from each projection angle into a plurality of image voxels, and thus make an image or an image volume with a single pass over the projection data. A classical resolution of the image generated by applying the filtered back-backprojection is based upon a size of the detector array, a size of a focal spot, a sampling rate of a data acquisition system (DAS) in sampling the analog projection data, and a kernel of a filter that filters the projection data during the filtered back-projection. In a typical scenario, the classical resolution is no finer than the size of a projection of each detector cell at an isocenter of the CT imaging system. By the Nyquist theorem, it is not necessary to sample at more than twice the limiting classical resolution. However, the image volume generated by the conventional direct image reconstruction techniques does not typically have a high spatial resolution.